cgfs: do not automount if cgroup namespaces are supported

cgfs: do not automount if cgroup namespaces are supported

In that case containers will be able to mount cgroup filesystems
for themselves as they do on a host.

This fixes inability to start systemd based containers on cgns-enabled
kernels with cgmanager not running.

I've tested debian jessie, busybox, ubuntu trusty and xenial, all of
which booted ok.  However if there are some setups which require
premounted cgroupfs (i.e. they don't mount if they detect being in
a container), this may cause trouble.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>